1. Field of the Invention
The present invention relates to aircraft control devices and more particularly to aircraft control stick force transducers.
2. Description of the Prior Art
VTOL (vertical takeoff and landing) aircraft which derive their VTOL capability by the directed thrust of turbojet engines are characterized as being extremely unstable in their hover-to-forward flight and forward flight-to-hover and landing modes, thereby requiring a high sensitivity stability or control augmentation system for safe operation. The high sensitivity of such a pilot augmented stability augmentation system requires a control stick having a control stick force transducer that is sensitive only to control forces or commands that the pilot consciously intends to apply to the control stick to produce pitch and roll maneuvers. Therefore, the force transducer should be insensitive to inadvertent or unconscious grip handle moments about these axes as well as the vertical axis and also insensitive to acceleration forces to which the grip may be subject.
Control stick force transducers are of course not broadly new, but known prior art attempts to develop control stick force transducers have resulted in devices that can be characterized as being overly sensitive to the acceleration of the grip handle or overly sensitive to a moment at the point of force; such as when in applying a linear force to the grip, the pilot unintentionally applies a wrist twist moment. Many prior art control stick force transducers which are moment sensitive are counterbalanced about a pivot point to eliminate linear acceleration sensitivity, whereas control stick force transducers which are acceleration sensitive are designed to minimize moment sensitivity.
The control stick force transducers which is described in U.S. Pat. No. 3,729,990, issued May 1, 1973, to the Applicants' assignee may be characterized as being not only of the acceleration sensitive type but also of the type that is moment sensitive about the vertical axis. The control stick force transducer disclosed therein utilizes a first array of cantilevered mode beams and unidirectional pick-off means arranged to sense sheer forces in a first direction but not in a second direction orthogonal to the first direction and a second array arranged to sense sheer forces in the second direction but not in the first direction. With this arrangement, moments about the vertical axis will couple into from the individual arrays and produce undesired output signals. Since the control stick force transducer does not include a grip handle compensation mass, the control stick transducer is also unduly sensitive to any longitudinal or lateral aircraft acceleration forces acting on the grip handle producing undesirable signal outputs.
The control stick force transducer described in U.S. Pat. No. 2,895,086, issued on July 14, 1959, to the Applicants' assignee, is of a type which may be characterized as being moment sensitive. In the illustrated embodiment of that invention, the deflection of a pivoted member that is attached to the manual grip member is measured from a null position by a suitable electrical transducer which provides an electrical output signal proportional in direction and magnitude to any force tending to pivot the pivoted grip member. While the control stick force transducer is not sensitive to acceleration forces acting on the grip handle, it is unduly sensitive to unintentional moments applied about the pivot point such as those produced by unintentional twisting of the pilot's wrist.
Accordingly, there is a need for a more effective control stick force transducer which can be particularly useful in stability augmentation systems and which is moment insensitive about all its axes of motion and insensitive to acceleration forces acting on the grip.